Recently, a story about people mistaking a news story for the Facebook login page has received considerable media attention. It's currently being seen by many as justification for the recent trend in locking people out of their computers for their own protection - but anyone with even basic mathematical skills and a calculator should come to the conclusion that this story has been blown way out of proportion.

You are absolutely right that UIs need to degrade and not just be dumbed down. But with your comment at the end you give the impression that anecdotes count for nothing. I would argue that it is exactly such anecdotes--from others and from your own life--which form the basis of understanding interaction design problems. Most times, a balanced account of a variety of anecdotes will be able to guide a designer in the right direction--just as simple as it needs to be, and no simpler. It's only due to some designers' tuning out the needs of more experienced users that a "dumbing down" effect takes place.